See, as I understood there is NO DIFFERENCE either to use standard 1600x1200x72 mode or custom 1600x1200x72.
It is same.
If you want to call the refresh rate change over the nvidia panel with MP you will either use the custom resolution trick or the .bat file trick. I don't think that you can set c:\\windows\\system32\\rundll32.exe NvCpl.dll,dtcfg setmode 1 1920 1080 32 XX in the Action field directly unless you have a custom resolution already made.
Too bad that it doesn't work cause for me this did the trick. I had exactly the same symptoms with you. The easiest way to see if DRC is properly working is to start a movie and then stop it. If DRC is switching refresh rate back then it is not working. It should not revert back to any refresh rate after the stop command (unless you have specified a "default refresh rate).